Terephthalic acid (TPA) is commercially produced by oxidation of para-xylene in the presence of a catalyst, such as, for example, Co, Mn, Br and a solvent. Terephthalic acid used in the production of polyester fibers, films, and resins must be further treated to remove impurities present due to the initial oxidation of para-xylene. Some commercial processes remove impurities by isolating a crude terephthalic acid solid, dissolving the solid in water at high temperatures and pressures, hydrogenating the resultant solution, cooling and crystallizing the terephthalic acid product out of solution, and separating the solid terephthalic product from the liquid as discussed in U.S. Pat. No. 3,584,039, herein incorporated by reference. Colored impurities from the benzil, anthraquinone, and fluorenone families are hydrogenated to colorless products and leave the process with the terephthalic acid solid product and wastewater streams.
Still other methods of obtaining a terephthalic acid product suitable as starting material for the production of polyesters do not involve a hydrogenation step. A method of producing a terephthalic acid product suitable for polyester production is to completely or nearly completely react para-xylene in a multiple stage oxidation process.
In an embodiment of this invention, a unique and novel process is provided wherein a crude carboxylic acid composition is produced by the oxidation of an aromatic feedstock, typically p-xylene, and is further oxidized in a multiple step process in the presence of added steam in a post oxidation zone.
In an embodiment of the invention, the addition of steam to a post oxidation zone subsequent to the primary oxidation zone results in a higher quality post oxidation composition compared to conventional techniques. This is not intuitive because the oxidation process actually produces water, and the current practice has been to minimize that production of water in the oxidation process; and it is not recognized that using added steam in the post oxidation zone would also produce a product of low color.